For an actress who has rarely been off our TV screens for the past two decades, Dervla Kirwan is curiously embarrassed by most of her appearances. “I’d like to forget a lot of the work I’ve done,” she says.

Which ones could she be thinking of? After making her name as an angelic wartime barmaid in Ballykissangel, the long-running Sunday-night drama about a rural Irish Catholic community, Kirwan appeared in the sitcom Goodnight Sweetheart and Doctor Who, before carving a niche in crime thrillers and one-off psychological dramas. She has also kept up her stage work with acclaimed performances in revivals of Pinter and Priestley classics.

And today, Kirwan is just as recognisable by her voice as her face, having purred that “This isn’t just food…” in a series of advertisements for Marks & Spencer.

Shifting restlessly in her armchair when we meet, Kirwan says that, until recently, her career hadn’t always gone the way she wanted: “It’s been very frustrating. I’m an impulsive and impatient person, and it’s taken me a long time to get where I want to go.”

You might think that a return to primetime TV drama in Blackout, a three-part BBC One thriller with Christopher Ecclestone, would cheer her up, but the 41-year-old actress demurs. “No matter how well people think you’re doing, or no matter how far up this invisible ladder of so-called success, there’s no security.”

As one of the most popular actresses on television, and with a husband of five years who is widely considered one of Britain’s most sought-after – not to mention most handsome – actors, Rupert Penry-Jones (the star of Silk and Whitechapel), what exactly is bothering Kirwan? Is she worried about being a fortysomething in an industry that prizes youth?

“There is a definite lack of great material for women of my age, and there are a large number of tremendously talented actresses out there. I had my crisis at 30, so to me it’s an irrelevance that I’m now in my forties. I don’t let it play on my mind because I’m very happy with my age,” she insists, before adding: “This isn’t some kind of neurosis.”

Oh, no – this is a Dervla Kirwan neurosis.

“Rupert is as worried as I am about what is going to happen in the next three to five years. He’s riding high now, but who is to say what’s going to happen in 18 months’ time? You’re in and out of favour so quickly. What happens when an actor starts losing his hair? Not that my husband is – he’s got a good head of hair. But I’m sure many couples have those conversations all the time with their partners.”

Although clearly smitten with “Roo”, Kirwan smiles with familiar resignation when asked how she deals with other women swooning over her husband – notably a fawning Holly Willoughby, who blushed and writhed on her daytime TV sofa when interviewing him recently. “What’s the alternative?” Kirwan sighs. “Look, he’s a very fine actor and – oh God, we’re straying into very dangerous territory here – he’s a very, very lovely man. I’m a very lucky woman. There you go.”

Her clipped response suggests she’s not entirely comfortable with the adoring attention her husband attracts. But does she ever raise her anxieties with him? “I never discuss my troubles in a relationship,” she says. “It’s best to just keep those thoughts to yourself. Why inflict it on the other person? Better to write it down in a diary and look at it three days later and think, 'Oh God, I know why I was thinking that way: I was hormonally challenged.’ It’s cheap therapy.”

As a fortysomething actress, there are plenty of other things for Kirwan to fret about: creeping crow’s feet, sagging skin and pressure to succumb to plastic surgery, for starters. She admits to weighing up the pros and cons of having something done, “but I would become a very dull and boring person if I let it dominate my life. It comes back to the question that if there was consistent work for women of my age until we’re 70, then it would be all right to age.”

But does that mean she would ever go under the knife? “Of course not. A lot of people have cosmetic surgery because they’re scared, but it can’t make you 20 again. And as you can see from my forehead,” she says, waggling her eyebrows, Groucho Marx-style, “I can still move my face to Olympic proportions.”

For proof that artificial enhancements aren’t a necessary tool of her trade, Kirwan only has to look to her mother-in-law, Angela Thorne, who played Mrs Frobisher in To the Manor Born. “She was exceptionally talented, but also an exceptionally beautiful woman in her day – still is – and she’s never had anything done. She thinks it’s ridiculous, which I find encouraging.”

The two women’s relationship came under scrutiny earlier this year after Penry-Jones let slip that Kirwan and his mother had had a very public altercation when the three met for lunch at The Wolseley, the exclusive London restaurant. After a reportedly “massive” row, the women were said to have left without eating, and in separate taxis.

“That’s rubbish,” snaps Kirwan, before slowly admitting that, yes, there was an argument, of a “very personal” nature, that it “was to do with grief”, and that it happened at a “very delicate and a very painful time” for the family.

“And do you know what was wonderful about it? This story came out and I rang Angie, and we had a fantastic conversation about why it had happened. So I’m still talking to my mother-in-law. We have a good relationship, a very good relationship.

“Who hasn’t had an argument with their mother-in-law?” she adds. “Rupert and I have been married for five years and together for 12; if that’s the only argument we’ve ever had, I think that’s pretty good going.”

Four years ago, the couple decided to leave London to bring up their two children, Florence, 8, and six-year-old Peter, amid the rural calm of Hampshire. After a stabbing in their hitherto quiet corner of Battersea, they moved to a house near Hambledon, complete with border collie, cinema room and pool.

“After that stabbing, I never felt safe. When you have kids, you don’t want them to get hurt.”

So has the shift to the countryside had the desired effect? “It has de-stressed us. It’s very conducive to my personality. For someone who is so impatient, it’s a natural medication.”

Typically, though, there’s a caveat: “Because I don’t live there anymore, I’ve fallen in love with London again,” she beams. It’s the restlessness – the “not being content” – that Kirwan says keeps her hungry. “As long as you’re chasing something, that keeps you alive, right?”